1. Field of the Invention
The present invention relates to a kneading apparatus for producing a thermosetting resin molded product by using a pressurized fluid containing high pressure carbon dioxide, and a method for producing the thermosetting resin molded product based on the use of the kneading apparatus.
2. Description of the Related Art
In recent years, various types of injection molding methods and extrusion molding methods have been investigated, in which high pressure carbon dioxide such as supercritical carbon dioxide or the like is used. In the case of the molding method as described above, a fluid having an extremely high pressure is introduced into a molten resin, and hence it is possible to produce molded products having various functions. For example, in order to mutually compatibly dissolve polymers which are incompatible with each other, an injection molding method and an extrusion molding method are suggested for a polymer alloy, wherein molten resins and high pressure carbon dioxide are brought in contact and kneaded with each other in a plasticizing cylinder (Japanese Patent Application Laid-open No. 2003-94477 and Proceedings of 17th JSPP Symposium of Japan Society of Polymer Processing, 227 (2009)). In the molding methods as described above, the molten resin and high pressure carbon dioxide are brought in contact and kneaded with each other by means of a kneading apparatus which is provided with a screw contained in a plasticizing cylinder. On the other hand, a molding method is suggested, wherein supercritical carbon dioxide is introduced into a molten resin at an intermediate position of an extrusion machine having a vent portion in order to remove any hardly volatile component from the thermoplastic resin (Japanese Patent Application Laid-open No. 11-292921). Further, an injection molding method is suggested, wherein a molten resin of thermoplastic resin is injected and charged into a mold from a plasticizing cylinder, and then a pressurized fluid, which contains supercritical carbon dioxide and a functional material such as an organic metal complex or the like, is introduced into the mold to thereby produce a thermoplastic resin molded product including the functional material dispersed on a surface (Japanese Patent No. 3964447).
In the meantime, the solubility of high pressure carbon dioxide with respect to the resin is low. Therefore, in the case of the molding method including the step of singly bringing the molten resin and the pressurized fluid in contact with each other as described above, it is difficult to allow a large amount of high pressure carbon dioxide and the molten resin to be brought in contact and kneaded with each other. Therefore, when the functional material is used together with high pressure carbon dioxide, it is also difficult to introduce the functional material into the molten resin at a high concentration. From such a viewpoint, a method for producing a molded product is suggested, wherein an kneading apparatus, in which an introducing port for introducing a pressurized fluid is provided on an upper side surface of a plasticizing cylinder and a vent port is provided on the downstream side from the introducing port, is used so that a molten resin and the pressurized fluid containing high pressure carbon dioxide and a functional material are brought in contact and kneaded with each other in the plasticizing cylinder, and then the resin internal pressure of the molten resin is lowered before being injected and charged into a mold to separate only gasified carbon dioxide from the molten resin and discharge carbon dioxide from the vent port (Japanese Patent Application Laid-open No. 2009-298838). According to this molding method, it is possible to improve the concentration of the functional material to be introduced into the molten resin, while controlling the concentration of high pressure carbon dioxide in the molten resin.
However, in the case of the kneading apparatus as described in Japanese Patent Application Laid-open No. 2009-298838, a high pressure kneading zone and a pressure reduction zone are sealed by the molten resin itself, wherein the molten resin and the pressurized fluid containing high pressure carbon dioxide are brought in contact and kneaded with each other in the high pressure kneading zone, and the resin internal pressure of the molten resin is lowered to separate gasified carbon dioxide from the molten resin brought in contact and kneaded with the pressurized fluid in the pressure reduction zone. Therefore, gasified carbon dioxide tends to be discharged from the high pressure kneading zone via the pressure reduction zone to the outside of the plasticizing cylinder during the period of the contact and kneading as well. The resin internal pressure tends to be lowered in the high pressure kneading zone. As a result, it is difficult to perform the contact and kneading for the molten resin and the pressurized fluid while maintaining the high pressure state. Further, a problem arises such that high pressure carbon dioxide is easily vaporized from the pressurized fluid in the high pressure kneading zone during the contact and kneading.
In view of the above, a kneading apparatus is suggested, the kneading apparatus including a plasticizing cylinder and a screw which is arranged rotatably and movably back and forth in the plasticizing cylinder, wherein a through-hole, which penetrates through a first area and a second area, is bored through the screw in order to separate an interior of the plasticizing cylinder into the first area in which a molten resin and a pressurized fluid are brought in contact and kneaded with each other and the second area in which gasified carbon dioxide is discharged, and a poppet valve, which is openable/closable by a spring, is arranged for the through-hole (Japanese Patent Application Laid-open No. 2010-253703). According to the kneading apparatus as described above, when the pressure of the molten resin in the first area exceeds the pressing force of the spring, then the poppet valve is opened, and the molten resin in the first area is allowed to flow into the second area. Therefore, the molten resin, which exists in any arbitrary area in the plasticizing cylinder, can be maintained at a desired pressure.
However, in the case of the kneading apparatus which has, as the seal mechanism, the poppet valve that is openable/closable by controlling the pressure by means of the spring as described above, the molten resin does not pass through the poppet valve as long as the resin internal pressure in the first area does not exceed the spring pressure. Therefore, in the first area, the molten resin receives the flow resistance by the spring pressure even before the pressurized fluid is introduced. Further, if the dispersibility of the pressurized fluid in the molten resin is taken into consideration, it is desired that the high pressure state is maintained as long as possible during the contact and kneading. Therefore, when the kneading apparatus as described above is used, it is necessary that the poppet valve having a high spring pressure should be used so that the poppet valve is not opened during a predetermined period of the contact and kneading even when the interior of the first area is at a pressure which is provided by adding the pressure of the pressurized fluid to the pressure obtained before introducing the pressurized fluid. As a result, in the case of the kneading apparatus as described above, the plasticizing ability is not only lowered due to the spring pressure during the plasticization, but the plasticizing ability also tends to be lowered, because the first area and the second area are not communicated with each other as long as the resin internal pressure of the molten resin in the first area does not exceeds the high spring pressure during the contact and kneading as well. Therefore, when a resin having a high viscosity is used, the plasticization and weighing tend to be unstable. Further, in the case of the kneading apparatus as described above, the heated molten resin passes through the through-hole provided in the screw. Therefore, the spring, which is arranged in the through-hole, is deteriorated by the heat of the molten resin in the course of the use of the molding machine for a long period of time, and the spring constant tends to be changed. As a result, it is difficult to constantly maintain the pressure of the high pressure kneading zone for a long period of time. A problem arises such that the molding cannot be performed stably in the industrial production.